Bills head coach Sean McDermott said on Wednesday that Allen was "day-to-day" as he battles a right elbow injury sustained late in Sunday's 20-17 loss to the New York Jets.
ESPN has reported that Allen is dealing with an ulnar collateral ligament issue in his elbow, which saw him miss four games in 2018.
Allen was present at practice on Thursday and wore a sleeve on his right elbow, but did not participate.
Keenum would start against the Vikings if Allen, who has a 64.1 per cent completion rate for 2403 passing yards with 19 touchdowns and eight interceptions this season, is unavailable.
"I'm doing the same thing I do every week, I prepare to play every week, every day, every period, every practice, every meeting," Keenum told reporters.
"It's the same, just got a few more reps today. I feel great, feel great with the communication, the mindset of the team with [offensive coordinator Ken] Dorsey and the communication from Sean everybody and taking it a day at a time.
"I've been in all situations. Played a lot without any practice, know how to do that, step into the middle of the game. I've practiced some and then played, I've practiced very little and played. I've literally been in any situation you could probably think of throughout the week."
Keenum, 34, is a former Vikings quarterback who has plenty of NFL experience, having started 64 regular-season games.
The Bills traded for Keenum in March from the Cleveland Browns, having started two games in the 2021 season for a 2-0 record.